PR 20472, PowerPC64 ifunc confusion

PR 20472, PowerPC64 ifunc confusion

This patch fixes quite a lot of confusion in allocate_dynrelocs over
ifuncs.  Function descriptors make ELFv1 quite different to ELFv2.

	PR 20472
	* elf64-ppc.c (ppc64_elf_before_check_relocs): Tweak abiversion test.
	(readonly_dynrelocs): Comment fix.
	(global_entry_stub): New function.
	(ppc64_elf_adjust_dynamic_symbol): Tweak abiversion test.  Match
	ELFv2 code deciding on dynamic relocs vs. global entry stubs to
	that in size_global_entry_stubs, handling ifunc too.  Delete dead
	weak sym code.
	(allocate_dynrelocs): Ensure dyn_relocs field is cleared when no
	dyn_relocs are needed.  Correct handling of ifunc dyn_relocs.
	Tidy ELIMINATE_COPY_RELOCS code, only setting dynindx for
	undefweak syms.  Expand and correct comments.
	(size_global_entry_stubs): Ensure symbol is defined.
	(ppc64_elf_relocate_section): Match condition under which
	dyn_relocs are emitted to that in allocate_dynrelocs.